Is there a cite (rather than the related employer references in IRC 1.415-2(d)(6)) that expounds on the use of compensation from a prior employer?

Specifically, what ownership percentage of a prior employer must a 100% shareholder of a current employer own in order to count the compensation from the prior employer for 415?

These corporations never existed together on the same day, and therefore never constituted a controlled group, or affiliated service group.

I can only refer you to the Code and regulations under Code Section 414(l) for guidance. Were the organization's defined benefit plan's merged post ERISA (post 1974) into a single plan?

earthy

I don't think there is a bright line test. My opinion is that you can't use the prior comp history unless the "successor employer was in substance a continuation of the predecessor employer" or, better yet, was a "mere change of form or locale". As you noted you don't have to aggregate the employers since they were never part of a controlled group, so you can't apply the controlled group rules to get the result you want unless you try to invoke the withdrawn 414(o) regs. Make sure your document language contains all the predecessor employer language, and get a determination letter. You may want to recommend a PLR if your client can afford it and they want to take a safe approach.